This pragma reflects early attempts to incorporate Unicode into perl and has since been superseded.
It breaks encapsulation (i.e.
it exposes the innards of how the perl executable currently happens to store a string),
and use of this module for anything other than debugging purposes is strongly discouraged.
If you feel that the functions here within might be useful for your application,
this possibly indicates a mismatch between your mental model of Perl Unicode and the current reality.
In that case,
you may wish to read some of the perl Unicode documentation: perluniintro,
perlunitut,
perlunifaq and perlunicode.

The use bytes pragma disables character semantics for the rest of the lexical scope in which it appears. no bytes can be used to reverse the effect of use bytes within the current lexical scope.

Perl normally assumes character semantics in the presence of character data (i.e. data that has come from a source that has been marked as being of a particular character encoding). When use bytes is in effect, the encoding is temporarily ignored, and each string is treated as a series of bytes.

As an example, when Perl sees $x = chr(400), it encodes the character in UTF-8 and stores it in $x. Then it is marked as character data, so, for instance, length $x returns 1. However, in the scope of the bytes pragma, $x is treated as a series of bytes - the bytes that make up the UTF8 encoding - and length $x returns 2: